1. Field of the Invention
The present invention relates to a monostable multivibrator circuit, and more specifically to a re-triggerable monostable multivibrator including a time constant circuit having at least a capacitor and for generating a predetermined width of pulse.
2. Description of Related Art
Monostable multivibrators are widely used in digital integrated circuits. It is a general matter that the monostable multivibrator used in the digital integrated circuits have been composed of logic circuits mainly due to limitation of the kind and characteristics of elements which can be assembled in the integrated circuit.
Referring to FIG. 1, there is shown a typical conventional monostable multivibrator having a re-triggerable function. The shown circuit includes an input terminal IN receiving a trigger signal, and a set-reset flipflop 10 having an inverted reset input R connected to the input terminal IN and a Q output connected to an output terminal OUT. A Q output of the flipflop 10 is connected to one input 12A of a two-input NAND gate 12, which has an output connected to a discharge circuit 14 of the open collector type or the open drain type. An open collector or drain output of the discharge circuit 14 is connected to a connection node A or series connected resistor R.sub.EXT and a capacitor C.sub.EXT, which cooperates to form a time constant circuit. The node A is connected to an inverted reset input R of the flipflop 10 through an inverter 16.
Furthermore, the input terminal IN is connected to an input of another inverter 18, whose output is connected directly to a first input 20A of a three-input NAND gate 20. The output of the inverter 18 is also connected through a delay circuit 22 and an inverter 24 to a second input 20B of the three-input NAND gate 20. A third input 20C of the NAND gate 20 is connected to the Q output of the flipflop 10. An output of the NAND gate 20 is connected to a second input 12B of the two-input NAND gate 12.
With the above mentioned arrangement, a negative-going trigger signal is inputted to input terminal IN, the Q of the flipflop 10 is brought from a high level to a low level, so that an output signal appearing on the output terminal OUT is brought to a low level. Namely, a negative-going one-shot pulse signal starts.
At the same time, the Q output of the flipflop 10 is brought from the low level to the high level. Since the second input 12B of the NAND gate 12 is at the high level in an initialized condition, both the inputs of the NAND gate 12 become the high level, and therefore, the output of the NAND gate 12 is brought from the high level to the low level. Thus, the open collector or drain of the discharge circuit 14 is put in a high impedance condition, so that the capacitor C.sub.EXT will be started to be charged through the resistor R.sub.EXT.
When a time constant determined by the C.sub.EXT and the resistor R.sub.EXT has elapsed, a potential of the node A will exceed a threshold of the inverter 16, so that a low level signal is applied to the inverted reset input R of the flipflop 10. As a result, the Q output of the flipflop 10 is returned from the low level to the high level. Namely, the negative-going one-shot pulse signal terminates.
Simultaneously, the Q output of the flipflop 10 is returned from the high level to the low level, so that an electric charge stored in the capacitor C.sub.EXT will be discharged through the open collector or drain output path of the discharge circuit put in a conduction condition. As a result, the potential of the node A is returned to almost the ground potential, and the inverted reset input R of the flipflop 10 is brought to the high level, so that the monostable multivibrator is returned to the initialized condition.
If a trigger signal is inputted to the input terminal IN in the way of the charge of the capacitor C.sub.EXT, the output pulse appearing on the output terminal OUT is elongated in the following manner:
When the capacitor C.sub.EXT is being charged, the three inputs of the NAND gate 20 assumes the following conditions:
First input 20A : low level PA1 Second input 20B : high level PA1 Third input 20C : high level
In this condition, if a re-trigger signal is applied to the input terminal IN, the first input 20A of the NAND gate 20 is brought to the high level, and therefore, the output of the NAND gate 20 outputs the low level. As a result, without regard to the level of the Q output of the flipflop 10, the NAND gate 12 will assume the high level, and the open collector or drain output path of the discharge circuit 14 is brought to a conduction condition. Namely, the capacitor C.sub.EXT which has been in the charging way, is forcibly discharged, again.
After a delay time .DELTA.t given the delay circuit 22 has elapsed, the second input 20B of the NAND gate 20 is brought to the low level, so that the output of the NAND gate 12 is brought to the low level. Accordingly, the open collector or drain output path of the discharge circuit 14 is brought to a high impedance condition. Namely, the capacitor C.sub.EXT is re-started to be charged.
When the re-trigger signal on the input terminal IN is returned to an original level (the high level), since the first input 20A of the three-input NAND gate 20 is brought to the low level at a time which is earlier than a moment that the second input 20B is returned to the high level, the output of the NAND gated 20 is fixed to the high level.
The above mentioned monostable multivibrator is disadvantageous in the following points:
The discharge time of the capacitor C.sub.EXT, when a re-trigger signal is inputted, is determined by the delay circuit 22. The capacitance of the capacitor C.sub.EXT is large, a long discharge time is required. In such a case, therefore, it has been necessary to modify the delay time .DELTA.t of the delay circuit 22.
In addition, if a trigger signal is applied in the way of a series initializing operation, the charge is started in some case before the capacitor C.sub.EXT has not yet been completely discharged. As a result, the width of the output pulse will adversely vary. In order to avoid this problem, the maximum capacitance of the capacitor C.sub.EXT has been limited. In other words, the conventional monostable multivibrator has been limited in the extent of application due to the maximum capacitance of the capacitor C.sub.EXT.